One quality characteristic of the photolithography process is critical dimension (CD). According to physical knowledge, CD is a function of the input recipe and the exposure dose (E). The increment or decrement of the exposure dose will lead to a proportional change in CD. Another quality characteristic of the photolithography process is overlay. Overlay control involves measuring misalignment between two successive patterned layers on the surface of a semiconductor device. Overlay depends upon many parameters, such as rotation, translation in the x- and y-axes, magnification, and so on. An additional factor in overlay control is machine matching. Often, it is not possible to use a single machine such as a scanner or stepper to expose all critical layers. For example, in order to avoid contamination of layers, semiconductor devices using copper employ one scanner or stepper for front-end processes and another scanner or stepper for back-end processes. In another example, a twin stage scanner may be used. A twin stage scanner has two wafer chucks. A single wafer may be processed on both chucks. It is necessary to match the overlay alignment between machines or between separate chucks on the same machine.
For example, in a conventional process, a monitor wafer is prepared. A first photoresist layer is exposed on the first machine and patterned (silicon wafer need etched). A second photoresist layer is then exposed on a second machine. A number of alignment marks were exposed on the first photoresist layer on the first machine. By shifting some distance in the X or Y direction, the same patterns were exposed on the second machine with the same photo-resist. By measuring all these alignment marks on the scanner, we can calculate the position error between the first and second layers which represents the overlay error between the two machines or the two chucks. This may take 60 minutes of scanner machine time to read all the alignment mark data. The data is modeled and then used to update the machine constants on the second machine so that the second machine is aligned to the first machine. One etched monitor wafer is needed for each machine or chuck, resulting in a higher production cost. Machine time for reading the data is significant.
U.S. Pat. No. 6,489,068 to Kye reveals a method for checking overlay error between two masks where overlay error is determined by observing darkness characteristics of alignment marks using the naked eye. U.S. Pat. No. 6,535,774 to Bode et al discloses modeling a disturbance in overlay control to modify input parameters to a scanner or stepper. U.S. Pat. No. 5,978,085 to Smith et al describes a method of measurement for correction of an optical system. U.S. Patent Application 2003/0097198 to Sonderman et al discloses a feed-forward correction method using an integrated metrology tool where errors in overlay control are compensated for in subsequent steps. U.S. Patent Application 2003/0087192 to Gau et al, assigned to the same assignee as the present invention, teaches the use of a single tool for all critical dimension patterning steps on a wafer so that machine matching is unnecessary. U.S. Patent Application 2003/0036007 to Baggenstoss teaches correction of a pattern on a mask to compensate for overlay error.